With the increasing reliability of wireless systems, many industries are opting to replace old hardwired systems with wireless ones. Wireless systems offer lower cost, higher flexibility systems that can be deployed in some applications where wired systems would be too cumbersome. Despite the benefits of wireless systems, their deployment in industrial environments can be difficult. A high number of metal barriers such as walls, metal containers and pipes can block wireless signals from their intended paths. In some cases, these barriers can be avoided by using repeaters mounted on either side of the barrier with a hardwire connection passing through the barrier. However, in some cases the option to penetrate the metal barrier, or route wire around it, is not viable.
In these select cases, ultrasonic through metal communication can be utilized to transmit through the barrier. Using ultrasonic transducers, the systems turn the barriers themselves into communication channels. Prior work in this area has developed ultrasonic through metal communications systems that can transmit at data rates up to 15 Mbps, as well as some systems that can transmit over 30 W of power through metal barriers, but these systems are limited in their application. Prior work in ultrasonic through metal communication has focused on transmission across flat metal barriers, with transducers that are well aligned and bonded to the barrier.
The present invention relates to the design and development of an ultrasonic through metal communication system that can be deployed on cylindrical metal walls. In industrial environments, there are many situations where through metal communication may be desirable across non-flat barriers such as pipes or the walls of pressure vessels. The curvature of these barriers presents a number of challenges that affect not only the physical development of the system, but also the development of the communication schemes. The present invention provides an ultrasonic through metal communication system that is capable of transmitting data across a curved barrier. By identifying and studying the most significant challenges in transmission across curved systems, the system described herein is capable of achieving a 1 kbps data rate at minimum and provides a base understanding of the problems curved barriers present. This will allow for the advancements from prior work to be adapted to this system for transmission across curved systems.
Initial work on through metal communications began in the year 2000 with the development and testing of the “HullCom” Acoustic Modem Unit by Hobart et al. in “Acoustic modem unit,” OCEANS 2000 MTS/IEEE Conference and Exhibition, vol. 2, pp. 769-772, 2000. The HullCom system was developed for use on Volunteer Observer Ships collecting Seas Surface Temperature data for the National Oceanic and Atmospheric Administration. The device was utilized to transmit temperature readings across the hull of the ship from a thermosalinograph at the base of the hull, to the main bridge at the top of the ship. Due to the incredibly low sampling rate, there was no need for high-speed communication and the maximum verified data rate achieved by Hobart et al, was 20 symbols per second.
The HullCom system communicated data from within a sealed box and had a relatively high power requirement, which presented a challenge for maintaining the internal system. In order to simplify the challenge of powering a transducer within a sealed container, techniques for power transmission via ultrasonic waves were explored by a team of researchers at University of Nebraska lead by Dr. Yuantai Hu as described by Hu et al. in “Transmitting electric energy through a metal wall by acoustic waves using piezoelectric transducers,” Ultrasonics, Ferroelectrics, and Frequency Control, IEEE Transactions on, vol. 50, no. 7, pp. 773-781, July 2003. Here, a general mathematical model was developed for assessing the feasibility and efficiency of transmitting energy through a metal wall via ultrasonic waves. This model considered two identical piezoelectric transducers mounted on opposite sides of a flat metallic barrier. The work concluded that it is possible to transmit electrical power through a metal barrier utilizing thickness mode piezoelectric transducers. The most interesting result from this research was that the maximum power transmission efficiency is not achieved at the fundamental resonant frequency of the system, but rather at one of the higher harmonic frequencies. The exact harmonic with the maximum efficiency is not constant across all systems, but varies based on an assortment of system parameters.
In 2005, a team of researchers from the California Institute of Technology expanded on the work done by Hu et al. and developed a general network model for “Wireless Acoustic Electric Feed-troughs” (Sherrit et al., “Efficient electromechanical network model for wireless acoustic-electric feed-throughs,” Proc. SPIE 5758, Smart Structures and Materials 2005: Smart Sensor Technology and Measurement Systems, 362 (May 20, 2005)). Sherrit et al. took the pure mathematical model developed by Hu et al. and created a network equivalent circuit model. The network model not only allows for more accurate and realistic results by considering all of the mechanics and real world loss of the system, but also allows for future work to be done by considering connections to additional networked circuits. Sherrit et al. completed calculations with their model for a variety of transducer parameters, and their work followed the same trends initially observed by Hu et al.
In 2006, a team at Rensselaer Polytechnic Institute worked to apply the theory developed by Hu and Sherrit (see Saulnier et al., “P1G-4 Through-Wall Communication of Low-Rate Digital Data Using Ultrasound,” Ultrasonics Symposium, 2006, IEEE, pp. 1385-1389, vol. 2, no. 6, October 2006). The team experimented with three different physical system layouts/communication methods for transmission through a 15.4 cm thick steel barrier. The first method, which was named the “Double-Hop Approach”, utilized a total of four transducers with a designated Transmit and Receive transducer on each side of the barrier. The second method, called the “Reflected Pulse Approach”, utilized a single pair of transducers mounted on opposite sides of the wall. The external transducer communicated by generating pulses, while the internal transducer communicated by altering its impedance to attenuate the reflected signal. The third method, called the “Hybrid Approach”, fused the previous two methods. Two transducers were utilized externally, one designated for transmit and the other for receive, while a single internal transducer communicated by attenuating the reflected signal. Utilizing a simple PAM sinusoid with the “Hybrid Approach”, Saulnier et al. successfully achieved 2-way communication with a maximum data rate of approximately 500 bps. At this limit the echoes within the metal barrier begin to create significant ISI.
Further work at Rensselaer Polytechnic Institute focused on the use of power harvesting in an ultrasonic communication system. In 2007, Shoudy et al. published a paper entitled “P3F-5 An Ultrasonic Through-Wall Communication System with Power Harvesting,” Ultrasonics Symposium, 2007. IEEE, pp. 1848-1853, vol. 28, no. 31, October 2007) discussing the details of a communication system that transmitted at 55 kbps with power transmission of over 0.25 W. This system utilized the “Reflected Pulse Approach” discussed by Saulnier et al., which involves two transducers mounted directly opposite each other on a metal barrier. The external transducer is powered directly and is an active system, while the internal transducer is passive and must be powered through ultrasonic energy before communication can occur. Utilizing a voltage doubling rectifying circuit, the internal transducer collects and stores energy from continuous wave (CW) transmissions until the system can start up. Once the system is fully powered, the internal transducer transmits data by adjusting its impedance and attenuating the reflected signal, which is then read by the external transducer. The maximum data rate of 55 kbps achieved in this paper is directly limited by the echoes created within the metal channel, which cause significant ISI.
The continuous echoes within the metal channel were the limiting factor in data rate for Saulnier et al., and are the most significant challenge to overcome for reliable through metal communication. In 2007, a team at Drexel University developed a method for reducing interference from echoes (Primerano et al., “Echo-Cancellation for Ultrasonic Data Transmission through a Metal Channel,” Information Sciences and Systems, 2007. CISS '07. 41st Annual Conference on, vol. 14, no. 16, pp. 841-845, March 2007. Utilizing a pair of transducers mounted directly opposite each other on a flat metal barrier, Primerano et al., developed a pre-distortion filter to significantly reduce the effect of echoes in the metal channel. Utilizing narrow pulses to encode data with a simple on-off keying (OOK) scheme, a maximum data rate of 50 kbs was achieved before ISI caused by echoes became too great. Through the development of a simplified system model, Primerano et al. created a pre-distortion filter that utilized a negative pulse to attenuate the echoes. This method and simple OOK modulation scheme allowed for a maximum data rate of over 1 Mbps.
Continuing work at Drexel University focused on the development of more sophisticated data modulation schema, while continuing to work with the same physical system consisting of two identical piezoelectric transducers, precisely aligned and securely mounted on opposite sides of a ¼″ steel barrier. Proper preparation of the metal surface ensures no rust and minimal surface contamination, while a layer of coupling gel is utilized to allow for more efficient energy transmission from the transducer to the metal barrier. Using this same physical system, Primerano et al. developed a better system model and pre-distortion filter (Primerano et al., “High bit rate ultrasonic communication through metal channels,” Information Sciences and Systems, 2009, CISS 2009. 43rd Annual Conference on, vol. 18, no. 20, pp. 902-906, March 2009), explored the use of OFDM (Primerano, R. “High Bit-rate Data Digital Communication through Metal Channels,” Ph.D. dissertation, ECE Dept., Drexel University, Philadelphia, Pa., 2010; Bielinski et al., “Application of Adaptive OFDM Bit Loading for High Data Rate Through-Metal Communication,” Global Telecommunications Conference (GLOBECOM 2011), 2011 IEEE, vol. 5, no. 9, pp. 1-5, December 2011; and Wanuga et al., “High-data-rate ultrasonic through-metal communication,” Ultrasonics, Ferroelectrics, and Frequency Control, IEEE Transactions on, vol. 59, no. 9, pp. 2051-2053, September 2012), and advanced bit loading algorithms (Bielinski et al., “Bit-Loaded PAPR Reduction for High-Data-Rate Through-Metal Control Network Applications,” Industrial Electronics, IEEE Transactions on, vol. 61, no. 5, pp. 2362-2369, May 2014) to achieve data rates up to 15 Mbps.
Further research at Rensselaer Polytechnic Institute resulted in a system that is capable of transmitting both data and power simultaneously. The final system was capable of transmitting data at a maximum rate of 12.4 Mbps while simultaneously transferring 32.5 W of AC power through a 6.3 cm block of steel (Lawry, “A high Performance System for Wireless Transmission of Power and Data through Solid Metal Enclosures,” Ph.D. dissertation, Dept. Elect. Eng., Rensselaer Polytechnic Institute, Troy, N.Y., 2011).
These papers represent significant advancements in the capabilities of ultrasonic through metal communication systems in the past 14 years. However, this prior research has only considered ultrasonic through metal transmission across flat metal barriers with piezoelectric transducers that are securely mounted to cleaned and prepared surfaces. In considering applications of through metal communication in industrial environments, there can be additional physical obstacles such as curved surfaces, inability to securely mount transducers, non-clean surfaces, etc. The focus of the present invention is on the development of an ultrasonic though metal communication system that can be utilized to transmit data across cylindrical metal walls.
Types of Piezoelectric Transducers and Transmission Methods
In looking to design a through-metal transmission system that can handle these non-ideal physical conditions, a variety of transmission methods were considered. These methods were based on the types of commercially available piezoelectric transducers. Piezoelectric transducers can be purchased from a variety of companies such as Steiner & Martins Inc. and APC International (Steiner and Martins, Inc., http://steminc.com; APC International, Ltd., https://www.americanpiezo.com), and are primarily used for non-destructive testing (NDT). Piezoelectric transducers are designed in a variety of shapes that each allow for excitation of different modes of acoustic wave propagation, which each have their own advantage for NDT. However, for the purposes of through metal communication, only three primary transducer types were considered in this project: Longitudinal mode, Shear mode, and Radial mode.
Longitudinal Mode
The first piezoelectric transducer type considered was longitudinal mode. Longitudinal mode transducers (FIG. 1) are the standard type of transducer used in most of the systems tested in the prior art. These transducers work by expanding and contracting through their thickness, normal to the metal barrier. This expansion and contraction generates compression waves that travel through the thickness of the metal.
Shear Mode
The second type of piezoelectric transducer considered was shear mode. These transducers (FIG. 2) expand and contract through their width, parallel to the metal barrier. This creates compression waves that travel along the surface and near surface depth of the barrier, as well as shear mode acoustic waves that travel through the barrier.
Radial Mode
The final type of piezoelectric transducer considered was radial mode. These are cylindrical transducers (FIG. 3) that expand and contract through their radius. This type of transducer has been utilized previously for underwater communication systems (Benson et al., “Design of a low-cost, underwater acoustic modem for short-range sensor networks,” OCEANS 2010 IEEE—Sydney, vol. 24, no. 27, pp. 1-9, May 2010). These ring transducers can be utilized to transmit waves through or along the surface of a barrier depending on how they are applied. These transducers also have the added benefit of a 360° transmission envelope, as opposed to the narrow directional transmission envelopes created by the other transducer types.
Each of these transducer types and the waves they generate have strengths and weaknesses based on the situation. Each must be given consideration in the development of a through metal communication system in order to develop the optimal set up. All three types are commercially available in a variety of common sizes, and can be custom ordered to fit the needs of a given system.
The most recent work on through metal communication noted above has focused primarily on increasing the maximum data rate of communication systems. While utilizing this common physical test bed of two identical transducers securely mounted on opposite sides of a flat metal barrier, recent focus has been on the development of more accurate system models and filters, the use of advanced modulation schemes such as OFDM, and different bit loading algorithms. In order to make ultrasonic through metal communication feasible across a wider array of industrial environments, efforts must be made to develop alternate physical layouts. In the present invention, an ultrasonic through metal communication system was developed that allows for communication across cylindrical metal walls.
Transmission across curved barriers presents a number of physical challenges that have not been considered in previous experiments. While the prior work by Lawry (noted above) cites transmission across a “slightly curved” metal barrier, there is no discussion by Lawry of the radius of curvature or any effects this had on his work, and is therefore presumed to be negligible. This work aims to explore transmission through surfaces with non-negligible curvature. Possible uses for this technology in industry include transmission through pipes or the curved surfaces of tanks and pressure vessels. In order to develop such a system there are three primary physical challenges that must be overcome: contact, curvature, and alignment.
Transducer Contact
The initial difficulty in deploying a through metal communication system on a curved barrier comes in the mounting of the transducers themselves. If attempting to utilize a flat longitudinal mode transducer on a curved surface, there would only be a single point of tangential contact externally, and two points of contact along the edge of the transducer internally (FIG. 4). This minor contact would not lend itself to a reliable communication system, so greater contact must somehow be established. The simplest solution to this problem would be to alter the geometry of the barrier to create two flat surfaces for mounting as shown in FIG. 4.
The primary drawback from this simple solution is that it requires manufacturing changes to the barrier, and in some cases this may not be possible or cost effective. Therefore, the inventors seek to develop a system that will allow for communication through a curved surface without need to significantly alter the geometry of the barrier.
Due to the challenges of getting proper transducer contact on a curved surface, the inventors have developed a system that does not require surface preparation or bonding of the transducers to the barrier. Current systems require cleaning and preparation of the barrier's surface, and solid mounting of the transducers with epoxy and coupling gel. In some industrial applications, this process may be cumbersome, or even impossible, due to factors such as rust, surface contaminants, or irregularities in the contour of the barrier. The ability to transmit without direct surface contact, or at least with non-bonded contact, would allow for ultrasonic through metal solutions in these non-ideal situations.
In looking for non-contact transmission methods, two alternatives were identified. The first option is the use of a laser for the creation of ultrasound waves. This method was first utilized in 1981 when researchers at the University of Hull in England explored the use of a Q-switched Nd:YAG laser to excite various ultrasonic wave modes for NDT applications in 1981 (Aindow et al., “Laser-generated ultrasonic pulses at free metal surfaces,” The Journal of the Acoustical Society of America, 69, 449-455 (1981)). This work successfully demonstrated the production of longitudinal, shear, and surface waves in a variety of metals using the laser, with no visible damage to the metal. While this method may be very effective and efficient in non-destructive testing, it would be impractical for most applications of through-metal communication. The development, installment and maintenance of a laser would be too involved for most through metal communication systems. Additionally, although the laser has the benefit of requiring no bonding to the barrier, it would still require significant surface preparation as the laser suffers losses in efficiency if the metal surface is not free from contamination.
A second option is the use of Electromagnetic Acoustic Transducers (EMATs). In 2009, Graham et al. at Newcastle University demonstrated the use of EMATS for successful ultrasonic through metal communication (Graham et al., “High bit rate communication through metallic structures using electromagnetic acoustic transducers,” OCEANS 2009—EUROPE, vol. 11, no. 14, pp. 1-6, May 2009). Utilizing two identical EMATs where each was held 0.8 mm from the surface of the barrier, Graham et al. were able to achieve a raw symbol rate of up to 40 ksps, which was improved to 1 Mbps with the application of a quadrature amplitude modulation (QAM) scheme. This presents a much more attractive solution since EMATs are significantly cheaper and easier to work with than lasers, and do not suffer the same efficiency losses from surface contamination.
Curvature/Echoes
The second challenge to overcome is the curvature of the barrier, and the effects it will have on the ultrasonic waves. The primary limiting factor on data transmission rates in through metal systems is the echoes, and the curvature of the surface has a significant effect on the propagation of echoes within the barrier. In order to develop an effective high-speed communication system, the echoes must be understood, and their effects mitigated. In order to understand how the curvature of the barrier effects the echoes in the system, an overview of acoustic echoes is presented below, and a series of acoustic simulations were completed, the results of which can be seen below as well.
Transducer Alignment
The final physical challenge addressed in accordance with the invention is the issue of alignment. When working with curved surfaces, especially on pipes and small pressure vessels, it may be difficult to guarantee proper alignment when installing through metal communication systems. As such it is important to understand the effects that transducer misalignment has on the system, and attempt to mitigate any negative effects. One way to do this is through the use of a hybrid communication system. In all of the prior work discussed thus far, a uniform pair of transducers has been used for data transmission. However, this may not always be the ideal system. In some applications, it may be impossible to align two longitudinal mode transducers across from each other, so it may be advantageous to utilize a longitudinal mode and shear mode transducer aligned orthogonally to each other. Or it may be preferable to have a securely mounted piezoelectric on the internal surface of a pressure vessel while using a non-bounded transducer such as an EMAT externally.
These three challenges are what makes transmission through a curved surface unique when compared with transmission across flat barriers. If these challenges can be appropriately addressed, the advancements made in prior research of through metal communications should be easily applicable to curved barriers.